1. Field of the Invention
This invention relates to a method of compensating for fluctuations in an image signal which are caused by nonuniformity in the intensity of a scanning light beam, nonuniformity in the speed at which a surface is scanned with the scanning light beam, nonuniformity in the sensitivity with which a photoelectric read-out means detects light emitted by a surface, or the like in an image read-out apparatus. In the image read-out apparatus the surface to be scanned, on which an image has been recorded, is scanned with a light beam in order to cause light carrying information about the image to be emitted from the surface, and the light is photoelectrically detected and converted into an image signal.
2. Description of the Prior Art
Image read-out apparatuses, wherein a surface on which an image has been recorded is scanned with a light beam, and light reflected by the scanned surface, light which has passed through the scanned surface, or light emitted by the scanned surface is detected in order to allow the image to be read out, have heretofore been used as, for example, image input means for computers and image read-out means for facsimiles. In such image read-out apparatuses, a light beam produced by a laser beam source or the like is deflected by a light deflector, which makes the light beam scan a surface, and light which is emitted by the scanned surface is detected and converted into an image signal by a photoelectric read-out means. By way of example, the photoelectric read-out means is constituted of a comparatively small photomultiplier and a light guide member, one edge face of which is positioned along a main scanning line on a surface to be scanned and the other edge face of which is positioned such that it is in close contact with a light receiving face of the photomultiplier. Alternatively, the photoelectric read-out means is constituted of a long photomultiplier which is positioned along a main scanning line of the surface to be scanned, as disclosed in, for example, Japanese unexamined patent publication No. 62(1987)-16666, or a line sensor which is positioned along a main scanning line.
However, with the image read-out apparatuses described above, an image signal generated by the photoelectric read-out means fluctuates due to nonuniformity in the intensity of the scanning light beam, which nonuniformity is caused by nonuniformity in the reflectivity of a reflecting surface of the light deflector. The image signal also fluctuates due to nonuniformity in the speed at which the light beam scans a surface, which nonuniformity is caused by fluctuations in the speed at which the light deflector deflects the light beam. Moreover, the image signal fluctuates due to nonuniformity in how the photoelectric read-out means detects the light emitted from (or reflected by, etc.) the surface, which nonuniformity is caused by nonuniformity in how the light is guided by the light guide member along the main scanning direction, or by nonuniformity in the sensitivity of the long photomultiplier along the main scanning direction. In cases where the aforesaid nonuniformities cause the efficiency with which the light is detected to worsen (i.e. cause shading to occur) it becomes impossible accurately to detect the image which was recorded on the surface to be scanned.
In order to eliminate the aforesaid problems, as disclosed in, for example, Japanese unexamined patent publication Nos. 62l(1987)-47259 and 62(1987)-47261, the applicant has proposed apparatuses for eliminating the shading wherein the characteristics of the shading are detected in advance, and the image signal, the sensitivity of the photomultiplier or the like is corrected in accordance with the position at which the light beam is scanning in the main scanning direction. Thereby adverse effects from the shading are eliminated.
In the image read-out apparatuses, rotating polygon mirrors which have a plurality of reflecting surfaces may be used as the light deflectors. A rotating polygon mirror is advantageous over a galvanometer mirror in that the speed at which the light beam is deflected can be increased, which allows an image read-out operation to be carried out quickly.
However, the rotating polygon mirror has problems in that the reflectivity fluctuates among the reflecting surfaces and the inclination with respect to the rotation axis of the rotating polygon mirror and the distance therefrom differ among the reflecting surfaces. Also, the speed at which the rotating polygon mirror is rotated fluctuates periodically, so that the speed at which the light beam is deflected differs among the reflecting surfaces of the rotating polygon mirror. As a result, the intensity of light emitted by a surface which is being scanned with the light beam differs from what it should be. These problems cause the shading characteristics to fluctuate, and shading cannot be eliminated accurately with the conventional shading elimination method.